This invention relates in general to waveguides in photonic-crystals and, in particular, to a structure where dielectric waveguides are incorporated in photonic crystal slabs.
Photonic crystal slab structures are constructed by introducing strong two-dimensionally periodic refractive index contrast into a high-index dielectric guiding slab. See, for example, Two-dimensional photonic-bandgap structures operating at near-infrared wavelengths, by T. F. Kraus, R. M. DelaRue, and S. Brand, Nature 383, 699 (1996). With sufficient refractive index contrast in the vertical direction, such structures support an in-plane photonic band gap that lies below the light line. For more information, please see High Extraction Efficiency of Spontaneous Emission from Slabs of Photonic Crystals, by S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and E. F. Schubert, Phys. Rev. Lett. 78, 3294 (1997); and Three-dimensional control of light in a two-dimensional photonic crystal slab, by E. Chow et al, Nature 407, 983 (2000). This allows the structures to function as a fundamental substrate for large-scale integrated micro-photonic circuit applications. For photonic integrated circuits, an essential building block is the waveguide structure. In order to function as an effective information carrying channel, the waveguide should possess several necessary properties: It should have its dispersion curve lying within the gap region below the light line to ensure low loss propagation within the guide and around sharp corners. The waveguide is preferably also single-moded, possesses sufficient bandwidth to accommodate the incoming signal, and displays minimal dispersion within the signal bandwidth. In a photonic crystal slab, a waveguide is typically created by introducing a line defect into the periodic lattices. These structures have been studied extensively with experiments and three-dimensional simulations. See, for example, any one of the following references:
1. Linear waveguides in photonic-crystal slabs, by S. G. Johnson, P. R. Villeneuve, S. Fan and J. D. Joanopoulos, Phys. Rev. B 62, 8212 (2000);
2. Demonstration of highly efficient waveguiding in a photonic crystal slab at the 1.5-mm wavelength, by S. Y. Lin, E. Chow, S. G. Johnson and J. D. Joannopoulos, Opt. Lett. 25, 1297 (2000);
3. Waveguides and waveguide bends in two-dimensional photonic crystal slabs, by A. Chutinan and S. Noda, Phys. Rev. B 62, 4488 (2000);
4. Methods for controlling positions of guided modes of photonic-crystal waveguides, by M. Loncar, J. Vuckovic, and A. Scherer, J. Opt. Soc. Am. B 18, 1362 (2001);
5. Clear correspondence between theoretical and experimental light propagation characteristics in photonic crystal waveguides, by T. Baba, N. Fukuya and A. Motegi, Electron. Lett. 37, 761 (2001);
6. Extremely Large Group-Velocity Dispersion of Line-Defect Waveguides in Photonic Crystal Slabs, M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama, Phys. Rev. Lett. 87, 253902 (2001); and
7. Large-bandwidth planar photonic crystal waveguides, by T. Sondergaard and A. Lavrinenko, Opt. Commun. 203, 263 (2002); and
8. Light-propagation characteristics of Y-branch defect waveguides in AlGaAs-based air-bridge-type two-dimensional photonic crystal slabs, Y. Sugimoto, N. Ikeda, N. Carlsson, K. Asakawa, N. Kawai, and K. Inoue, Opt. Lett. 27, 388 (2002).
However, many of the proposed waveguide structures exhibit relatively small guiding bandwidth and large group velocity dispersion. Developing ways to enlarge the waveguide bandwidth is therefore an important direction of research in photonic crystal structures. For examples of such effort, please see Demonstration of highly efficient waveguiding in a photonic crystal slab at the 1.5-mm wavelength, by S. Y. Lin, E. Chow, S. G. Johnson and J. D. Joannopoulos, Opt. Lett. 25, 1297 (2000); Methods for controlling positions of guided modes of photonic-crystal waveguide, by M. Loncar, J. Vuckovic, and A. Scherer, J. Opt. Soc. Am. B 18, 1362 (2001); and Large-bandwidth planar photonic crystal waveguides, by T. Sondergaard and A. Lavrinenko, Opt. Commun. 203, 263 (2002).
None of the above-described approaches are entirely satisfactory. It is therefore desirable to provide an improved waveguide structure, a method of making the structure, with characteristics that are improved over those described above.